FIG. 21 (A) shows a DC/DC converter 91 of a step-up and -down type which runs with a current critical mode, and it shows a control device 92.
The DC/DC converter 91 consists of a power supply PSDC (it generates an input voltage Ei of the direct current), a transistor switch Tr, a reactor L, a diode D and a capacitor C.
One terminal of the power supply PSDC is connected to one terminal of the transistor switch Tr.
Another terminal of the transistor switch Tr is connected to one terminal of the reactor L and a cathode terminal of the diode D.
Another terminal of reactor L is grounded.
An anode terminal of the diode D is connected to one terminal of the capacitor C and one terminal of a load R.
Another terminal of the capacitor C is grounded, and another terminal of the load R is grounded, too.
In FIG. 21 (A), a resistance (a resistance for measurement) rL to measure a reactor current (a current iL which flows through the reactor) is connected to the reactor L in series.
The control device 92 comprises a control circuit 921 and a driver 922.
The control circuit 921 inputs an input voltage (an input voltage E) of the DC/DC converter 91 and an output voltage eo and a reactor current iL, and a control in the current critical mode can be performed.
In the current critical mode, the reactor current iL changes in a form of a saw wave pattern (or a shape that was similar in a shape of a saw wave pattern). In the current critical mode, a lowest current value of the reactor current iL is zero (or a value which is almost zero).
The control device 92 measures a voltage (a reactor voltage vL) between two terminals of the reactor L.
And the control device 92 calculates the time when iL changes to zero based on vL/L (=m1) and (eo−vL)/L (=m2); wherein “vL/L” is a slant of the reactor current iL in an ON period (t1−t2), “(eo−vL)/L” is a slant of the reactor current iL in an OFF period, and the time when iL changes to zero means a timing for turn-on.
As above, a control by the current critical mode is performed.
FIG. 22 (A) shows another DC/DC converter 93 of step-up and -down type and another control device 94.
In FIG. 22 (A), the reactor L is provided with a secondary winding TL for a measurement.
The control device 94 comprises a control circuit 941 and a driver 942.
The control circuit 941 inputs an input voltage (input voltage E) of the DC/DC converter 93, an output voltage eo of that and a reactor voltage vL of that.
The DC/DC converter 93 can perform a control with the current critical mode in this way.
As shown in FIG. 22 (A), a reactor voltage vL is measured as a measurement voltage vL′.
The control circuit 941 calculates a time when the reactor current iL becomes zero, based on formula “Ldi=−vLdt”.
FIG. 23 shows an AC/DC converter (a switching power supply) 95 and a control device 96.
A technology to detect the reactor current iL is used in the system in FIG. 23.
In FIG. 23, a rectifier circuit RCD which inputs AC power to the input side of an AC/DC converter 95 is comprised.
The control device 96 comprises a control circuit 961 and a driver 962.
The control circuit 961 includes a third control part (a power factor improvement part) 9611.
By a control quantity which the third control part 9611 generates the AC/DC converter 95 runs with a current critical mode.
A power factor of the AC/DC converter 95 is improved in this way.